Oxidation is the core technology to convert petrochemical based materials to commodity chemicals of higher oxidation state. However, oxidation reactions present some of the greatest challenges to the environment. Many of the industrial oxidation processes involve heavy metals or corrosive reagents, and are high in energy consumption.
Selective benzylic oxidations of alkyl substituted aromatics to ketones or carboxylic acids are fundamentally important laboratory and commercial organic transformations1,2,3. The oxidation products are essential intermediates for the manufacture of high-value fine chemicals, agrochemicals, pharmaceuticals and high-tonnage commodities4,5. Such benzylic oxidations can be achieved by various oxidizing agents based on transition metals, such as permanganate and chromate. If molecular oxygen, hydrogen peroxide or hydroperoxide are used as the oxidants, metal-based catalysts either homogeneous or heterogeneous are needed2,6.
For example, terephthalic acid is produced by oxidation of p-xylene under aggressive conditions. Terephthalic acid is ranked as one of the top 50 industrial chemicals with a global demand amounting to 40 million tonnes per year and an annual growth rate around 5 to 10%6. It is the key monomer of polyethylene terephthalate (PET). PET is the common material for making soda bottles and other packaging resins. Textile and synthetic fibres such as polyesters (Terylene) are also made from terephthalic acids.
In the industrial process of making terephthalic acid, p-xylene is oxidized by air in acetic acid at 200° C. and a pressure of 20 atm. The catalyst system is made of bromides of heavy metals and salts of cobalt and manganese. The reaction must be carried out in a titanium-lined reactor because the reaction mixture is highly corrosive. On the other hand, gas-phase oxidation of o-xylene to phthalic acid is carried out with fluidized-bed V2O5 based catalyst. Phthalic acid is the key precursor of many plasticizers. In all these known oxidation processes of methyl aromatics, metal based oxidizing agents or catalysts are required. If air is used as the oxidizing agent, high reaction temperature and pressure are usually involved.
In recent years, there have been considerable efforts for the development of environmentally conscious procedures for the production of carbon and aromatic feedstocks7,8,9. For instance, the uses of heavy metals should be avoided due to their toxicities, negative environmental impacts and the need for removal of residual metals from the reaction products. The uses of organic solvents should also be reduced, if not totally abandoned, to reduce VOC emission. This so called Green Chemistry is said to be the future of a sustainable chemical industry.